This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The Multicenter AIDS Cohort Study is a prospective cohort study that was begun in 1984 to follow HIV seropositive and seronegative men. The UCLA Natural History of AIDS Study is one of four centers of the Multicenter AIDS Cohort Study. There are 1637 subjects enrolled at UCLA, and 5622 MACS-wide. There are now about 1800 active subjects enrolled. Subjects are followed semi-annually who undergo administration of the MACS-wide questionnaires, a battery of neuropsychologic tests, a brief physical examination, and the routine collection of blood and possible collection of semen, saliva, urine, or stool samples. Supplemental funding from NIAID was awarded to open recruitment in order to improve statistical power and enroll Latino and African-American minorities, as well as younger (<39 yrs) Caucasian men, the groups currently most affected by HIV/AIDS, with the goal of enrolling ethnic groups reflective of the current epidemic. Total enrollment of subjects at Harbor GCRC is estimated at about 240 men who will each have two visits per year.